Wells are drilled into the earth's crust to desired subterranean formations, e.g., oil- and/or gas-beating formations, through the application of rotary drilling techniques. In the rotary drilling of a well, a drilling fluid is circulated through the well in order to remove the cuttings therefrom. The drilling fluid is pumped downwardly through a rotating drill string within the well and thence upwardly to the surface of the well through the annulus surrounding the drill string. The drill cuttings are entrained in the drilling fluid and withdrawn from the well with the fluid. In addition to removing cuttings, the drilling fluid also serves other functions such as lubricating the drill string and bit, cooling the drill bit, and providing sufficient hydrostatic pressure down hole to prevent the flow of formation fluids into the well.
In most rotary drilling procedures the drilling fluid takes the form of a "mud," i.e., a liquid having solids suspended therein. The solids function to impart desired rheological properties to the drilling fluid and also to increase the density thereof in order to provide a suitable hydrostatic pressure at the bottom of the well. The drilling mud may be either an aqueous base mud or an oil-base mud. While oil-base muds are used most advantageously where the formation being drilled is a suspected oil or gas producer, it usually is desirable to use a water-base mud throughout most of the drilling operation. The water-base mud may be a freshwater mud, a salt-water mud, or an emulsion mud formed by an oil-in-water emulsion in which the water is the continuous phase.
One difficulty which has long been experienced with the use of water-base drilling muds is the deleterious effects of the water in the drilling mud on certain formations penetrated by the drilling procedure. For example, so-called soft shales, which are relatively unstable clay-containing formations, upon contact by the aqueous drilling fluid tend to hydrate such that sloughing occurs. This sloughing or influx of the shale or other earth formation into the well during the drilling operation often leads to serious and costly problems, such as stuck drill pipe, undesired hole enlargement, and even such extreme instability as to cause loss of the well because of inability to proceed with the drilling operation. Desirably, the mud forms a thin permeable coating on the walls of the borehole to reduce loss of water from the borehole to the formation, thereby reducing softening of the borehole walls and sloughing into the hole.
In water-based drilling muds, days are used to provide desirable viscosity and filtration properties. However, in the course of the drilling operation, clays from formation solids that become dispersed because of borehole instability can be a major problem in certain formations. Drilling in gumbo type formations, high in montmorillonite, can be troublesome and expensive.
Clay mineral particles have internal chemical defects that produce localized negative charges on the surfaces of the platelets. These negative charges are electrostatically balanced by cations surrounding the clay particles and forming ionic clouds around them. These cations are characterized as exchangeable cations of the clay particles because they can be exchanged with other ions under certain conditions, such as by contacting the clay particles and aqueous solutions of other ions. The electric charges on clay particles generate electrostatically repulsive forces whereas the mass of clay particles give rise to van der Waals' attractive forces. The net result of these attractive and repulsive forces determines whether the clay particles will be in a coagulated or dispersed state when in contact with an aqueous fluid. The smaller the cationic cloud around each particle, or the more contracted the electric double layer, the more effective the neutralization of the localized negative charges on the surface of the clay platelets will be. Effective neutralization of the negative charge on clay platelets by cations tends to cause coagulation or shrinking of clay particles. Expansion of the electric double layers of clay platelets leads to swelling and dispersion of clay particles.
The sensitivity of gumbo clay containing formations in contact with fresh water is increased by exposure to monovalent ions such as sodium and hydrogen. The primary reason for the dispersion of clays is expansion of clay particles due to hydration of the sodium cations which are adsorbed between platelet faces. The readily hydratable sodium ions adsorbed on the clay particles attracts a large number of molecular layers of water causing expansion between clay platelets and thus weakening the van der Waals' forces tending to hold the particles together.
The prior art involves various techniques which have been proposed in order to alleviate the problems associated with clay hydration and swelling. Such techniques involve the use of anionic or cationic suffactants, the use of oil slugs interposed in the water-base mud, and the use of salts such as sodium or calcium chloride.
One class of additives for use in inhibiting clay hydration is based upon polyamine structures carrying various functional groups which work to render the clays resistant to swelling with the attendant problems of sloughing and even plugging of the borehole and damage to desirable oil or gas formations. For example, U.S. Pat. No. 2,761,843 to Brown discloses a clay treating mechanism in drilling operations which is described as involving an ion exchange reaction between exchangeable cations on the clay and substituted ammonium ions in the treating solution. Here, substituted ammonium ions are derived from a basic aliphatic polyamine such as diethylenetriamine (DETA), triethylenetetramine TETA), tetraethylenepentamine (TEPA), and various other higher molecular weight polyamines.
Another procedure for use in the formulation of clay inhibiting drilling fluids is disclosed in U.S. Pat. No. 2,873,251 to Jones. Here various polyamine salts are formed in situ within a drilling fluid by the addition of a polyamine such as triethylenetetramine with a reactant such as an organic acid, e.g. acetic acid, followed by the addition of a thickening agent such as carboxymethylcellulose. An optimum treatment is described in Jones as involving about two pounds per barrel of triethylenetetramine, 3 pounds per barrel of acetic acid, and four pounds per barrel of sodium carboxymethylcellulose. As in the patent to Brown, various other polyamines are disclosed as useful precursors.
Similarly, polyamine based drilling fluid additives and their use are disclosed in U.S. Pat. No. 3,127,344 to De Groote. Here a wide range of polyamines are disclosed as starting materials including polyalkylene polyamines as described above and in addition cycloalkyl and aromatic polyamines. The amines may be characterized as nonhydroxlated or hydroxylated alkylene polyamino mines ranging from diethylenetriamine to tetrabutylenepentamine, used either singularly or in mixtures. Various cyclic amines include heterogenous 5 and 6 member rings. Acylating agents which can be reacted with the base polyamine product include carboxylic acids which can contain functional groups such as hydroxy groups. Alpha hydroxy acids such as glycolic acid (hydroxyacetic acid) and lactic acid, are particularly disclosed. Polycarboxylic acids can also be used.
Additional polyalkylene polyamine products useful as drilling mud additives, as well as in various other applications such as emulsifiers and deemulsifiers, are disclosed in U.S. Pat. No. 3,200,106 to Dickson et at. The products disclosed in Dickson are derived from branched polyamines by various reaction routes involving acylation, alkylation, oxyalkylation and various combinations thereof, such as acylation followed by oxyalkylation and the corresponding reverse procedure, oxyalkylation followed by acylation. Other products are derived by olefination and carboxylation in which the branched chain polyamine precursor is reacted with ketones or aldehydes such as formaldehyde. Suitable acids useful in the acylation procedures include mono- and poly- carboxylic acids containing other functional groups such as hydroxyacetie acid.
Various drilling fluid additives useful in controlling clay swelling are also disclosed in U.S. Pat. No. 5,149,690 to Patel et at. Here, the drilling fluid additives are formed by the reaction of an aliphatic polyamine such as diethylenetriamine or triethylenetetramine with a polyfunctional aliphatic acid such chloroacetic acid or hydroxyacetic acid as described above. Upon completion of the reaction between the amine and the acid, potassium hydroxide is added to the resulting product to raise the pH to about 7.0.